| Summary: | Luman (CREB3, LZIP) is a basic leucine zipper transcription factor involved in regulation of the unfolded protein response (UPR), dendritic cell maturation, and cell migration. But despite reported expression in primary sensory neurons, little is known about its role in the nervous system. Luman mRNA from rat sensory neurons was amplified and its coding sequence was determined. The rat Luman cDNA contains a full-length open reading frame encoding 387 amino acids, and the recombinant protein generated from this clone activated transcription from UPR elements. Quantitative RT-PCR revealed rat Luman transcripts in a variety of rat tissues with the highest levels in nervous system tissue. In situ hybridization confirmed the findings and demonstrated that the Luman mRNA hybridization signal localizes to neurons and satellite glial cells in dorsal root ganglia (DRG), the cytoplasm of hepatocytes in liver, and the hippocampal pyramidal cell layers in CA1 and CA3 and the granular cell layer of the dentate gyrus. Luman protein localizes with axonal endoplasmic reticulum (ER) components along the axon length within the sciatic nerve and is activated by sciatic nerve injury. Adult sensory axons also contain Luman mRNA which is translated within the axon and transported to the cell body via the importin-mediated retrograde transport system in response to nerve injury. Further, creation of an N-terminal, C-terminal dual fluorescence-tagged Luman adenoviral construct allowed visualization of the cleavage and retrograde translocation of the N-terminal portion of Luman to the nucleus in real time in vivo and in vitro. Neuronal or subcellular axonal knockdown of Luman significantly impaired the intrinsic ability of injury-conditioned, but not naïve, sensory neurons to extend the regeneration-associated elongating form of neurites. Sciatic nerve crush injury also induced activation of the UPR in axotomized DRGs, including genes linked to cholesterol biosynthesis. Knockdown of Luman decreased the activation of UPR and cholesterol biosynthesis, and axotomy-inducted increases in neurite outgrowth, which could be largely rescued with either mild UPR inducer treatment or cholesterol supplementation. Together these findings provide novel insights linking remote injury-associated axonal ER responses to the regenerative growth capacity of adult sensory neurons via axonal activation and synthesis of Luman and reveal a role for the UPR in regulation of axotomy-induced neurite outgrowth that is critically dependent on Luman.
|
|---|
